Meditation for Kids (and Other Beings), by Laurie Fisher Huck
This is a tiny book – literally pocket sized, and if I count right, 72 pages including the copyright pages & about the author info.
It’s a really spiffy book, with a great introduction to meditation and an explanation of what meditation is, with text & illustrations that appear to be hand drawn. It’s witty, but down to Earth, and easy to read and understand.
For what it’s worth, this isn’t a “how to meditate” book, so much as a “why meditate” book. You’ll probably need to do some follow up with your children to actually get them meditating.
The views expressed here are my own. I bought the book new several years ago, and the Amazon link is an affiliate link.